three things you HAVE to enable to virtualize a 64 bit os. (assuming your machine has 64 bit hardware and os)
1. enable IO APIC (settings ((control-s))>general tab>advanced)
2. enable VT-x/AMD-V (settings ((control-s))>general tab>advanced)
3. use one of the Intel Ethernet adapters (the pc-net drivers are 32bit only, causing the virtualized system to not boot)
without these things (which are all backwards by default) a 64 bit os just wont work. im running winxp 64 just fine with these settings.